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RS-485 / CAN Termination & Bias Planner

Size RS-485 fail-safe bias resistors, sanity-check termination loading, and get practical CAN termination values (classic + split) with quick notes.

Fail-safe bias - Termination sanity checks - Split termination notes

Quick start (2 minutes)

  1. Select the tab: RS-485 (A/B) or CAN (CANH/CANL).
  2. Set termination: for a long trunk or higher bitrates, keep 2 ends and set the resistor to the cable impedance (usually ~120 Ω).
  3. RS-485 only: set receiver count + unit load (from the transceiver datasheet), then pick a target idle Vdiff (start at ~250 mV). Implement one bias network (one location) and avoid parallel networks.
  4. CAN only: defaults are usually enough. Use split termination when you need better EMC (midpoint capacitor to GND).

This is a first-pass planner. Always cross-check your transceiver thresholds, internal failsafe features, and common-mode range in the datasheet.

Do you actually need RS-485 bias resistors?

The RS-485 tab is for sizing an external fail-safe bias network (one pull-up + one pull-down) to force a stable idle Vdiff when nobody is actively driving the bus.

You may NOT need external bias if...

  • Your transceiver guarantees receiver output for open inputs / shorted inputs ("failsafe", "true failsafe").
  • Your system never leaves the bus floating (someone always drives a known idle state).
  • You have a modern transceiver with built-in fail-safe behavior (verify in the datasheet).

You likely NEED external bias if...

  • All drivers can be disabled (Hi-Z) and you still need a deterministic idle state (common with UART framing).
  • You are using older RS-485 parts without a guaranteed idle/open-circuit behavior.
  • The bus can be disconnected / partially unplugged and you want a defined default state.

If you add bias: do it in one location only. Multiple bias networks in parallel can overload the bus.

Datasheet checklist

  • Search for: failsafe, open-circuit, idle, receiver threshold.
  • Look for a table/statement that the receiver output is defined when A/B are open, shorted, or near 0 Vdiff.
  • If it is not guaranteed, use the RS-485 tab to create a positive idle Vdiff margin.

RS-485 A/B polarity is vendor-dependent. Always verify the datasheet definition (do not rely on connector labels).

Pick what you are wiring. Use RS-485 when your bus is an A/B multi-drop that can float when all drivers are disabled. Use CAN for CANH/CANL; it usually only needs termination (no external bias network).

Tip: if you already know you are on a CAN bus, jump straight to the CAN tab and keep termination count at 2 ends.

RS-485 fail-safe bias

RS-485 termination & bias

Size one pull-up + one pull-down network (one location) to keep a stable idle Vdiff when nobody is driving the bus. Many modern transceivers include internal "failsafe" behavior; if yours does, you may not need external bias.

What to enter

  • Set termination first (ends + resistor value), then adjust receiver assumptions (node count + UL).
  • Target idle Vdiff is a margin choice: 200–300 mV is a common starting point.
  • Implement one external bias network on the bus (avoid parallel networks).

Typical twisted-pair Z0 is ~100–120 Ω.

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References: TIA/EIA-485 (RS-485), ISO 11898-2 (high-speed CAN), and vendor app notes on RS-485 fail-safe biasing and CAN split termination.

What this planner assumes

RS-485 bias model

The tool assumes one external fail-safe network made of equal pull-up and pull-down resistors (Rpullup = Rpulldown = Rbias). It models the differential load as the parallel of terminations and receiver input resistance.

The math used is the symmetric divider: Vdiff = VCC * Rload / (2 * Rbias + Rload).

CAN termination model

CAN classic termination assumes two resistors equal to cable impedance Z0 placed at the two physical ends. The tool reports the expected “ohmmeter value” (≈ Z0/2) and the dominant-state current based on a typical dominant Vdiff.

Split termination keeps the same differential termination while adding a midpoint capacitor to reduce common-mode noise.

Wiring checklist (fast)

  • Terminate only at the ends of the main trunk (don’t terminate every node).
  • Keep stubs short (the stub is usually what turns a “works in the lab” into a field failure).
  • One RS-485 bias network: avoid multiple “failsafe” pull networks in parallel.
  • Twisted pair: route the differential pair together and keep return paths consistent.
  • Check transceiver datasheets for input thresholds, internal failsafe features, and common-mode range.

Frequently asked questions

When do I need RS-485 fail-safe bias resistors?

Only when the bus can be left floating (no driver actively driving) and your transceiver does not guarantee a stable receiver output for open/short/near-zero Vdiff conditions. Many modern RS-485 parts include internal failsafe behavior, so external bias is often optional. If you add bias, implement one network on the bus (avoid parallel networks).

Where should I place termination resistors on RS-485 and CAN?

Terminate only at the physical ends of the main trunk. Do not terminate every node. Keep stubs short, especially at higher bitrates or when drivers have fast edges.

Why does my CAN bus measure about 60 ohms across CANH and CANL?

With two 120 ohm terminators (one at each end), the effective resistance across CANH-CANL is 120||120 = 60 ohms when the bus is unpowered.

What is CAN split termination and when should I use it?

Split termination uses two resistors per end (R/2 + R/2) and a capacitor from their midpoint to ground. Differential termination stays ~Z0, while the capacitor helps shunt common-mode noise for better EMC.

Why are RS-485 A/B labels confusing?

RS-485 naming varies across vendors. Some datasheets label the non-inverting pin as A, others as B. Always cross-check the datasheet definition (or the sign of Vdiff) instead of assuming A/B polarity from the connector.

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